Inhibition of FGF Activity

Angiogenic growth factors, like FGF-2 and VEGF165, require interaction with HS in order to induce a proliferative signal through tyrosine kinase receptors. This represents a novel target for inhibiting growth factors involved in angiogenesis. Compounds that block the interaction of these angiogenic factors with HS were found to block endothelial cell proliferation and angiogenesis in vivo.

Suramin, a polyanion, disrupts binding of FGFs to their low- and high-affinity receptors (99). It has been used in clinical trials on cancer patients, with some promising effects. However, suramin is not specific for FGFs and also interferes with binding of several other growth factors to their receptors. Also, treatment of KS derived cells with suramin or protamine had no inhibitory effect. Instead, it resulted in the upregulation of FGF-2, FGF-5, and FGF receptor, consistent with clinical observations that suramin caused stimulation of KS growth in patients (100). A limitation on the clinical use of suramin is the narrow margin between the dose required to achieve antitumor effect and that leading to the onset of prohibitive toxic side effects. Several polysulfonated naphthylureas, with structures related to suramin, effectively blocked FGF-2-stimulated growth of capillary endothelium in vitro and FGF-2-driven angiogenesis in vivo (101). These, together with their lower toxicity, offer the opportunity of widening the suramin therapeutic window.

Pentosan polysulfate, another polyanionic heparin analog, has been reported to inhibit proliferation of tumor cell lines derived from breast carcinoma, prostate carcinoma, lung carcinoma, epidermoid tumors, and rhabdomyosarcomas, in culture and in athymic mice (102). Human adrenal cancer cell lines that have been transfected with FGF-4 were dramatically inhibited in their proliferation and tumor formation by pentosan polysulfate, when suramin or dextran sulfate exhibited only a slight inhibitory effect (103). However, treatment of HIV-associated KS in patients with pentosan polysulfate did not reveal a significant tumor response (104). The antitumoral effect of suramin, pentosan polysulfate, laminarin sulfate, and other polysulfated compounds was also attributed to their inhibition of heparanase activity, and hence tumor metastasis (105).

Tecogalen (DS4152), a low-mol-wt peptidoglycan extracted from the bacterial wall of the bacterium Arthrobacter, appears to inhibit angiogenesis by interfering with binding of FGF-2 to endothelial cells. It is currently being evaluated in a phase I clinical trial for the treatment of refractory malignancies, including breast, lung, and head and neck cancers. In addition to its antiangiogenic activity in vivo, it also has antitumor activity in vitro and in vivo (95,106).

5.1.1. Heparin-Mimicking Compounds

In an attempt to identify potent mimetics that can modulate abnormal FGF signaling, the authors have synthesized a series of negatively charged, nonsulfated aromatic compounds that mimic many of the effects of heparin (62,107,108). These nontoxic, nonsulfated polyanionic compounds were found to compete with HS on the cell surface and ECM, on FGF-2 binding (Fig. 1). Compound RG-13577 (polymer of 4-hydroxyphenoxy acetic acid and formaldehyde ammonium salt, Mr ~5,800) and related compounds were also found to revert the transformed phenotype of FGF-2-transfected cells through a disruption of FGF-2-mediated autocrine loop (107). Direct interaction between compound RG-13577 and FGF-2 was suggested by the ability of the former to release FGF-2 from ECM or heparin-Sepharose (107), and to compete with heparin on binding to FGF-2 (62). Compound RG-13577 also inhibited FGF-2-receptor binding, as demonstrated in crosslinking experiments (62). Unlike heparin, compound RG-13577 alone failed to induce dimerization of FGF-2. Moreover, it abrogated the dimerizing effect of heparin (62). Similarly, dimerization of FGFR on the surface of HS-deficient CHO cell mutants was prevented when the cells were incubated with FGF-2 and heparin in the presence of excess compound RG-13577. These effects were associated with a profound inhibition of FGF-2-mediated signal transduction (tyrosine phosphorylation) and proliferation of vascular endothelial and smooth muscle cells (62,108). Moreover, the antiangiogenic effect of compound RG-13577 was demonstrated by its ability to inhibit the outgrowth of microvessels from rat aortic rings embedded in a collagen gel (62). Synthetic polyanionic compounds that block the interaction of FGFs and other heparin-binding growth factors with HS may therefore provide a potential avenue for pharmacological intervention with undesirable effects of heparin-binding growth factors in processes such as tumor angiogenesis, diabetic retinopathy, and restenosis of balloon injured blood vessels (63).

Biological activities of FGFs can also be modulated through the respective high-affinity receptors. One approach is the utilization of dominant-negative receptor constructs, which, when co-expressed with FGF receptors, can block activation and signal transduction. Some of these tools have already been employed to study the functional role of FGFs and their receptors in airway epithelia development, and in wound healing (109,110). In a similar manner, NIH 3T3 cells transformed by FGF-4 were suppressed by expression of tyrosine kinase-deficient dominant negative FGFRs (111).

Another approach to interfere with tumor growth is the ligand-specific targeting of toxins to tumor cells expressing FGFRs. For example, a fusion protein consisting of FGF-2 and saporin, a cell toxin isolated from the plant saponaria, was specifically targeted to FGFRs, and exhibited antitumor activity in vitro and in vivo (112). Recombinant versions of these fusion proteins expressed in Escherichia coli inhibited growth of B16-F10 melanoma cell lines in vitro, and retarded tumor growth and metastasis in vivo (113). Alternatively, fusion of FGF-1 to Pseudomonas exotoxin A resulted in specific cytotoxicity to a variety of tumor cell lines expressing FGFRs, including those of prostate, colon, or breast carcinoma (114). Systemic application in athymic mice grafted with several different tumor cell lines slowed tumor growth, but did not induce complete regression. Probably, the exotoxin attacked the tumor mass, rather than the tumor endothelium (114). Pseudomonas exotoxin FGF-1 fusion proteins also inhibited endothelial cell proliferation and tube formation in vitro, by inducing dose-dependent cell death (115).

Other inhibitory compounds have been developed that bind and inactivate FGF ligands. However, their specificity toward FGFs and/or other growth factors is not yet established. For example, using systemic evolution of ligands by exponential enrichment (SELEX), high-affinity RNA ligands to FGF-2 have been isolated that inhibit binding of FGF-2 to its receptor (116). It has also been reported that phosphothioate oligodeoxynucleotides bind to FGF-1, -2, and -4, thereby preventing their binding to FGFRs, and removing FGF ligands from low-affinity binding sites on the cell surface and ECM (117). Based on a multitude of expression studies in tumor biopsies and cell lines, it is well accepted that FGFs and their receptors appear to play a role in the growth regulation of many cancers. Experiments utilizing tools that interfere with FGF ligand and receptor activity clearly demonstrated such a functional role. However, better and more specific reagents need to be developed in order to efficiently interfere with either tumor cell proliferation or tumor angiogenesis.

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